The wording of:

> To opt-in, paste "{your username}.at.hn" anywhere in your profile's 'about' section.

Is slightly incorrect, I think.

Everyone seems to visible by default, opted-in or not.

However, the ?refresh thing is locked down so you need to opt-in before. But by default, every user page is accessible on at.hn, even if you don't have the link in your profile.